Present day communication systems, whether they are used for transmitting analog data or transmitting digital data, employ high power microwave amplifiers, such as solid state power amplifiers and travelling wave tube (TWT) amplifiers, as part of the signal transmission or transponder sections of the system. Unfortunately, such power amplifiers have extremely non-linear AM/AM and AM/PM conversion characteristics that cause serious distortion of multi or varying amplitude signals passed through them. This distortion is a primary impediment to reliable spectrally-efficient digital signalling through such an amplifier.
FIG. 1 is a graph of the typical non-linear conversion behavior of a microwave power amplifier. In the FIG. 1, curve 1 is a plot of output signal power and curve 2 is a plot of output phase shift, each curve plotted relative to input signal power for a sinusoidal signal. As can be seen from the curves presented in FIG. 1, the variations of both output signal power and phase shift of the amplifier are non-linear over a considerable range of input signal power.
One way to avoid the distortion effects exhibited in FIG. 1 for digital modulation signalling would be to use constant envelope type signals, such as unfiltered PSK or FSK modulated signals. Unfortunately, such signalling schemes generally require a higher signal-to-noise ratio for a prescribed level of performance than those types of modulation (such as QAM) that employ variations in amplitude to represent the data. The performance disparity between constant and non-constant amplitude signals becomes larger as the number of bits/sec/Hz is increased. Accordingly, if the performance efficiency of a non-constant amplitude signal modulation scheme is to be obtained, the AM/AM and AM/PM distortion characteristics such as illustrated in FIG. 1 must be compensated.
One brute force approach for reducing the effects of such distortion would be to reduce the drive level into the amplifier, so that the amplifier output power is considerably below saturation (power amplifier back off) where the magnitudes of the AM/AM and AM/PM distortion are tolerable. While this technique has been found to be useful and has been widely employed with TWT amplifiers, it loses a great deal of its appeal if the amplifier has to be backed off excessively in order to obtain acceptable distortion levels, since every dB of amplifier back off causes a loss in a dB of radiated power. For high data rate per Hz systems, acceptable distortion levels are generally in a region of 35 to 50 dB down and, in order to obtain such levels, the amplifier must be backed off excessively.